1. Field of the Invention
The invention relates to both the delivery of particulate solids to a reaction chamber and to internally mixing the particulate solids and fluid, principally feed, at the reactor, and is particularly well adapted in an apparatus and process for use in the Thermal Regenerative Cracking (TRC) process, as described in U.S. Pat. No. 4,061,562 to McKinney et al and U.S. Pat. No. 4,097,363 to McKinney et al.
2. Description of the Prior Art
Particulate solids are used in gas phase or liquid phase reactions for a variety of reasons. Typically, the particulate solids are present to catalytically accelerate (or rarely, decelerate) the rate of reaction. In still other reaction systems, solids are admixed and reacted with the fluid reactants. During the course of the reaction the solids participate in the reaction as a reactant and are depleted. Another use of particulate solids is to supply heat for the reaction. Hot inert solids are added to the reaction zone simultaneously with the gaseous reactants, the heat being transferred to the fluid medium by direct heat transfer. Conversely, the particulate solids may occasionally be employed to remove heat of reaction.
Typically, fixed bed and fluidized bed reactors are used to contact the solids with reactants. However, in reactions where reaction residence time is low, tubular reactors are used to create plugged flow velocity profiles. Such profiles prevent backmixing of the reactants, and ensure uniform reaction radially along the length of the tubular reaction zone. Gradients normal to the flow of material through the tubes are undesirable because such temperature and concentration variations interfere with the yield and distribution profiles from the reaction.
An example of the use of solids as a heat supply medium is illustrated in the above mentioned U.S. Pat. No. 4,061,562 to McKinney et al which describes a TRC process to react residual petroleum oils to produce olefins, particularly ethylene. In this system the reaction proceeds axially along the length of the riser reactor, the temperature gradually decreasing as endothermic heat of reaction is withdrawn therefrom.
In each of the systems described above, as well as other systems requiring the continuous addition of particulate solids, the means of feeding the solids to the reaction zone should satisfy two criteria. First, the flow of solids must be regulated. Generally, the solids flow rate is adjusted to reestablish the set point of a controlled parameter, such as temperature, pressure, density, concentration of a particular product in the effluent, and the like. In conventional thermal cracking processes, for example, it is common to control furnace coil outlet temperature to +/-10.degree. F. This is equivalent to a +/-2% variation in the heat input. Thus, McKinney's process would require control of the solids feed rate to approximately +/-2%. The requirements for control of the flow rate of solids is influenced by the reaction time. For reaction times of less than one second, flow must be stable on a micro second level since the reaction times are less than the typical response times of conventional control means i.e., valves, etc.
Second, the solids feeding device should allow rapid and complete mixing of the solids and fluid phases. This is especially true where reaction residence times are low, as in thermal cracking. If mixing takes an appreciable percentage of the reactor residence time, concentration and temperature gradiants normal to the flow of material passing through the early stages of the reaction zone will create variations in reaction rate. Hence, product yields and distribution will be affected adversely. The present invention is an apparatus and a method for effecting flow control and uniform mixing of a particulate solids stream when introduced into a fluid stream.
Heretofore solids flow rate has been regulated in several ways using pneumatic or mechanical means. One method and system for facilitating the delivery of particulate solds to a reactor is to fluidize the entire bed of solids in the chamber feeding solids to the reactor. This system is attended by the use of valves and other regulation means to regulate the gas pressure above the bed.
However, fluidized beds characteristically have poor control over bed height so that control of the flow through the orifice is similarly poor. The bed has fluid characteristics and the fluctuations in bed height, bed density, and overhead pressure are transmitted uniformly throughout the fluidized bed essentially instantaneously. Hence, the pressure above the orifice constantly varies, and the resultant variations in orifice pressure differential cannot be compensated for because recovery time is often too long. Thus, variations in solids flow to the reaction zone is inherent in this flow control system.
A second method used to deliver solids to a tubular reactor relies on pneumatic transport gas injected into a lift leg located between the reservoir and the reactor. By varying the transport gas flow rate to the lift leg, the density of material in the lift leg is regulated thereby controlling the back pressure through the orifice which provides communication between the lift leg and the reservoir. This system has the disadvantage of generating gas bubbles in the lift leg which produces fluctuations in the solids flowrate.
Control is further compromised in this system because the high rate of aeration gas necessary to transport the solids is a negative influence on the rapid and uniform mixing of the solids and feed streams in the mixing zone at the top section of the reactor. In addition, large quantities of transport gas entering the reactor necessitate the use of over-sized reaction chambers to accommodate the inert aeration gas medium.
The third method employs mechanical valves to physically alter solids flowrate. Generally, valves are single or double disk types, the latter being preferred where uniformity of mixing is desirable. However, there is considerable erosion of the valve seat by the solids in each of these valves. These valves, therefore, must be replaced frequently, and have other maintenance problems associated with sealing the valves and properly maintaining and ascertaining the variable response that occurs as the valve seat erodes.
None of these three methods is particularly compatible with the operation necessary for the rapid and uniform mixing of the phases in low residence time reaction systems. Indeed, in order to feed solids to a TRC ethylene reactor, as noted above, the flow must be controlled to within .+-.2 percent or cracking severity oscillations will be greater than that presently experienced in coil cracking. The solids feed device (local fluidization) of the subject invention minimizes bed height as a variable and dampens the effect of over bed pressure fluctuations, both of which contribute to flow fluctuations. It is thus uniquely suited to short residence time reactions, especially on the order of 0.05 to 2 seconds, as in the above mentioned TRC systems.
Further, for short residence time reactions, the rapid and intimate mixing are critical in obtaining good selectivity (minimize mixing time as a percentage of total reacting time).
Both of the above mentioned features permit the TRC to move to shorter residence times which increase selectivity. Conventional fluid bed feeding devices are adequate for longer time and lower temperature reactions (FCC) especially catalytic ones where minimal reaction occurs if the solids are not contacting the gas (poor mixing).